1. Field of the Invention
The present invention relates generally to a record medium useful in optical reading and writing of high-density information, and more particularly to improved fabrication methods for optical disks.
2. Description of the Prior Art
Optical disk recording media have various configurations, and may be rigid or flexible. Rigid optical disks are made with either an optically clear plastic or glass substrate or a polished aluminum substrate. Plastics are generally used for low-capacity disks with low data rates, whereas aluminum and glass substrates are generally used for high-capacity, high-data-rate disks.
The currently preferred optical disk technology employs disk elements with spiral or concentric tracks of minute (usually on the order of a micron or less in size), optically detectable marks. One real-time mode of recording (writing) such marks is by scanning tracks on the disk with an equivalently small beam of radiation (e.g., from a laser), which is modulated "off" or "on" according to an electrical signal representative of the information to be written. Information is recovered (read) by scanning the tracks with a larger, but still very tightly focused, radiation (e.g., light) spot. The recovered information is in the form of a fluctuating electrical signal obtained from a photodetector that senses the read-out light reflected from the recorded disk.
In order to write and read information in the form of such minute markings, optical systems of high numerical aperture are used to focus light to equivalently minute spots. Such optical systems have extremely small depths of focus, and the proper positional relation between the writing or reading optical system and the optical disk record surface must be stringently maintained. Therefore it is highly desirable that the optical disk support surface underlying the record layer be smooth (i.e., relatively free of high-spatial-frequency variations from a nominal plane, e.g., minute pits or bumps) and flat (i.e., relatively free of large-amplitude, low-spatial-frequency variations, e.g., undulating surface variation of the support). Although complex focus-servo devices can effect lens adjustment to compensate for imperfect smoothness and flatness, such devices add to the cost and fragility of the write/read apparatus. The required complexity of the focus-servo devices is proportional to the degree of such disk imperfections and the speed of operation.
One approach to achieve requisite smoothness and flatness has been to form the disk substrate of glass with a ground and polished surface. That requires a time-consuming and costly fabrication procedure. Another approach is to start with a disk substrate with a generally smooth surface and apply a surface smoothing sub-layer by spin-coating techniques. Smoothing sub-layers applied by spin-coating techniques improve the surface characteristics, but still exhibit substantial high-spacial-frequency variations.
Besides the requirement for smoothness, most rigid optical disks require some tracking feature to be a permanent part of the disk. This is referred to as "preformatting" and is generally either a groove or an optically written data track. Grooving the disk is the fastest method, and for plastic substrates is done either during the process of molding the plastic substrate or during a subsequent sub-layer casting operation, such as the "2p" process described hereinafter. For aluminum substrates, tracking features are generally optically written onto disks, but that requires up to one hour for a two-sided, 14-inch-diameter disk.
FIG. 1 is a cross-sectional view of a portion of a typical prior-art disk 10. Each disk half has a transparent substrate 12 coated on one side wth a primer layer 14 and a featured (i.e., having a pattern of depressions and/or protuberances), molded sub-layer 16. Sub-layer 16 may contain a data track containing video picture and sound information. A thin, reflective metal mirror coating 18 is applied to molded sub-layer 16 so that information may be read from the disk by optical reflection. The mirror coating is in turn coated with a protective layer 20.
Manufacture of such known optical disks begins with the making of a mold. A typical mold-making process is shown in FIGS. 2a to 2g. A polished glass blank 22, FIG. 2a, is coated with a layer of photoresist 24, FIG. 2b. Picture and sound information is written onto the photoresist layer by means of modulated laser light focused by an optical system 26 (FIG. 2c) so that it is registered in the form of a pattern of voids in the developed photoresist layer (FIG. 2d). After application of a release layer (not shown), a thin nickel layer 28 (FIG. 2e) is electrochemically deposited onto the patterned side of the glass blank and is then removed to form a single metal negative copy of the vulnerable master, which is destroyed in the process, this copy being referred to in the industry as the "father." Electrochemical copying of the father produces a limited number (six or so) of "mother" copies 30 (FIG. 2f) before the father has deteriorated to the point that the resolution of additional mothers would be unsatisfactory. The mothers are positive copies of the master, and are themselves copied a limited number of times to produce negative "son" copies 32, shown in FIG. 2g. The sons are used as molds for mass production. This "family" process is necessary so that many production molds may be made from a single glass master mold. However, each generation of the family, and each succeeding member of the same generation, suffers increasing resolution degradation.
Several replication methods are usable with the molds which result from the previously described process. For example, optical disks may be molded by a compressive technique wherein a preheated plastic mass is forced against the featured mold under high pressure. The high pressure, temperature, and rate of cooling can cause some deformation of the mold so that local non-circularity of the tracks is introduced. There may also be residual stresses in the plastic, producing undesirable birefringence and/or a warped disk.
Another method for replicating disks involves injection molding. In this method, granulated plastic is melted and injected under pressure into a mold cavity. The disadvantages of this approach are comparable to those of compression molding.
In yet another replication method, a heated mold is used to impress the information at high pressure and temperature into the surface of a plastic disk at room temperature. In principle, only the surface of the disk is affected in this process. However, since it is difficult to obtain disks that are sufficiently flat, the disk must be preheated so that complete plastic deformation is possible, leading to the disadvantages mentioned earlier with respect to compression and injection molding.
Still another method known in the prior art is based on a photopolymerization process, which is generally referred to as the "2p" process. In this process, a liquid composition of monomers of acrylates (esters of acrylic acid) is polymerized on a featured mold by exposure to ultraviolet radiation to form a featured sub-layer. FIGS. 3a to 3d illustrate steps in the 2p process. A few milliliters of the 2p liquid (34 in FIG. 3a) are applied to the center of a metal, featured mold 30 which has been prepared as described hereinbefore. A transparent substrate 12 , coated on one side with a primer layer 14, is placed on the mold such that the space between the mold 30 and the coated substrate 12 is filled by a layer of 2p liquid 34 (FIG. 3b). This layer is exposed to ultraviolet light (FIG. 3c) to polymerize the 2p liquid and form sub-layer 16. The 2p material of sub-layer 16 does not adhere to the mold 30 but does adhere to the coated substrate 12; and after the exposure, the substrate and the cured record layer bonded thereto are removed from the mold as shown in FIG. 3d. A reflective metal mirror coating 18 and a protective layer 20 (FIG. 1) can then be applied to the sub-layer 16.
While the 2p process does not share the above-noted disadvantages inherent in the compression molding, injection molding, and embossing techniques, it does suffer from the resolution degradation which comes from making multiple-mold generations, and is usable only when the substrate is transparent to the radiation used to cure the 2p liquid.
Several materials have been proposed for the substrate used in the 2p process. Such materials include polymethyl methacrylate (PMMA), polyvinylchloride (PVC), and polycarbonate (PC). It has been found that aluminum disk substrates are more desirable than substrates of plastic material. Because a departure from flatness in the disk causes vertical displacement of the information track, any such departure complicates the read/write operations. Disk flatness can be better maintained with aluminum. Similar considerations apply to radial deviations of the information from the ideal. Compared to the three transparent materials proposed for the substrate of the 2p process, a disk substrate of metal material would be superior for holding tolerances.
Other problems exist for plastic substrates. PMMA is sensitive to moisture and requires an adhesion layer before application of the 2p material because cured 2p coating does not adhere well directly to a PMMA surface. Absorption of water vapor causes changes in the mechanical properties of PMMA, which can lead to warping of the disk. PVC has only marginally acceptable birefringence properties, exhibits dimensional stability vulnerability at high temperature, and is subject to attack of the mirror coating by additives and decomposition products. PC poses problems of birefringence, has a high molding temperature of about 140.degree. C., and has a tendency to stress-crack when in contact with 2p liquid.